Fluoroscopy



Dec. 2, 1969 .Y. PAL'r A 3,432,093

FLUOROSCOPY Filed Nov. so. 4lease FIG. 1

BRIGHTNESS ENHANCEMENT FIG. 2 1.5.

Loi

o'5500 250 |25 IOO 75 60 50 O 30 YORAM PALTI United States Patent O3,482,093 FLUOROSCOPY Yoram Palti, Jerusalem, Israel, assignor to YissumResearch Development Company, Hebrew University, Jerusalem, acorporation of Israel Filed Nov. 30, 1966, Ser. No. 597,986 Claimspriority, application Great Britain, Dec. 3, 1965, 51,446/ 65 Int. Cl.G01n 23/04; H01j 37/22 U.S. Cl. 250-53 1 Claim ABSTRACT F THE DISCLOSUREBACKGROUND Fluoroscopy of humans by means of X-ray is very fequentlyused in various diagnostic and surgical procedures. Recently the use ofthis convenient and important examination tool has been drasticallycurtailed because of the fear of cumulative effects of radiation, whichmay cause severe damage. As the method of uoroscopy is of considerablediagnostic value, any method which is adapted to decrease the amount ofradiation applied to the examined person is of considerable value.

During the conventional uoroscopy of still patients some of theradiation applied to the patient reaches the examiner who may be exposedto dangerous cumulative effects. A similar problem arises in the X-rayexamination of welded, cast and other articles for the detection ofmanufacturing flaws. Any method which decreases the required output ofan X-ray machine, makes possible the use of such machines which aresafer and of simpler and less expensive construction.

In order to decrease the quantity of applied radiation, various opticaland electronic devices have been developed. Among these there may bementioned the use of a uoroscent screen in conjunction with an imageintensifier, adapted to result in an image of adequate brightness withsubstantially decreased intensity of radiation. Such intensifiers arerather expensive, and devices of equivalent performance, but ofsubstantially decreased cost are widely sought.

Conventioinal X-ray machines make use of continuous radiation (which maybe of the frequency of the current used, such as 50 or 60 cycles), andafter passing through the tissue the radiation impinges on a fluoroscentscreen, where an image is obtained which corresponds roughly to theintensity of the impinging radiation.

SUMMARY that the fluoroscope employed is a fluoroscope as aforesaid.

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The source of X-rays may be an X-ray tube provided with a grid or othercontrol electrode (eg. an electrode arranged to deflect the electronbeam away from the target) by which the generation of the X-rays isrendered intermittent, or by supplying the tubes anode with pulsatilepotential. For example, a high frequency pulse generator may be placedbetween the power source and the X-ray tube of the uoroscope.Alternatively the stream of X-rays may be interrupted with a mechanicalshutter. Advantageously the repetition frequency at which the X-rays areemitted in the direction of the screen and the time fraction of thecycle for which the X-rays are emitted in the direction of the screenare both variable. The intensity of the X-rays emitted in the directionof the screen is preferably also variable.

DRAWING FIG. 1 is a schematic circuit diagram of a pulse generatingcircuit for providing X-ray pulses in accordance with the presentinvention; and

FIGS. 2 and 3 are graphical representations of screen brightness basedon various pulse frequencies.

DESCRIPTION It will be noted that when the actual generation of theX-rays is rendered intermittent, all parts of the screen tend to beaffected simultaneously by the intermittent effect. However Where amechanical shutter is provided, there will often be a tendency for thedifferent parts of the screen to be affected in sequence; this isespecially true where the shutter is a rotary device having parts whichare substantially opaque (eg. 'formed of lead), and apertures or otherparts which are transparent, to the X-rays or having parts of markedlydifferent transparencies to the X-rays. A mechanical shutter in the formof a disc, with a variable speed drive, which has a transversely movableaxis of rotation and which is divided into transparent and opaque partsbounded by generally radial, but appropriately non-linear boundaries,may be employed to vary both the repetition frequency and the timefraction of the cycle for which the X-rays are emitted in the directionof the screen.

In place of a mechanical shutter, rotating disc or otherwise, anelectronic system may be used. Such a system may merely comprise avariable pulse generator in the X-ray machine circuit. Such a pulsegenerator may be located immediately before the X-ray tube in thecircuit, or, if desired, the entire power input to the X-ray machine maybe controlled by such a pulse generating circuit.

Various other alternatives are also possible including mechanicalswitching devices. For example, power to the X-ray tube or the machinemay be controlled with a rotating disc switch having electricallyconductive and insulating areas thereon and an electrically conductiveelement sliding along the surface of such rotating disc.

For convenience of description, reference will be made hereinafter topulses of the radiation. Actual pulses are obtained only when theradiation from the X-ray tube is cut off or reduced and then returned toits former level in each cycle. They are not obtained with themechanical devices such as shutters which do not affect all parts of thescreen simultaneously but it will be understood that references topulses are to be taken as including such shutters unless the contextotherwise demands.

In using the apparatus the frequency, duration and intensity of thepulses will be adjusted so as to result in a bright image, yet withsubstantial decreased total quantity of radiation as compared withconventional methods.

The brightness of the fluorescent screen depends on the intensity of theradiation and to a lesser degree on its fr equency, the eiciency of mostscreens being somewhat higher at higher frequencies. The total amount ofemitted light depends also on the total duration of irradiation. Itwould be expected that the use of pulses totalling say half the time ofobservation would result in an image of about half or slightly more thanthe intensity of continu ous radiation of equal intensity. This would befound to be true if the light emitted by the fluorescent screen weremeasured with photometric means, but the perception of light-intensity(brightness) by a human observer depends on the intensity of the lightand on its distribution in time. If the light is applied in pulses,there is obtained the physiological phenomenon called sommation,according to which the application of the stimuli results in an effectwhich is considerably greater than the time average of the individualstimuli. The sommation gives rise to a bigger and longer nervousresponse per pulse resulting in a perception by the observer ofincreased intensity and prolonged duration as compared with a continuousstimulus of integrally equal total radiation. The prolongation ofperception is usually due to the development of a repetitive response,in this case at some levels beyond the obscrvers receptors themselves.

There exists a critical fusion frequency (CFF), which varies betweenabout -80 pulses per `second depending upon the intensity of the light.At this, or at a higher frequency, there is obtained an apparentlycontinuous image. The frequency of pulses used for fluoroscopy accordingto the present invention is preferably above the CFF, so as to give theobserver the impression of a continuous image. It was foundexperimentally that when the duration of a light pulse is around 1millisecond and the pulse repetition rate is around 50 pulses/sec.(which is over the CFF for light intensities obtained by uoroscopicscreens) the sensation of brightness is enhanced (see FIG. 2). Suchpulses give a sensation of brightness equivalent to the brightness of acontinuous source of light, the energy output of which is from 2 to 3times larger than the integral output of the pulsatile light.

The pulse duration and frequency which give the best brightnessenhancement depend on the size and intensity of the light source, on thedegree of dark adaptation of the observer and on his degree of fatigue.For this reason the repetition frequency at which the X-rays are emittedin the direction of the screen and the time fraction of the cycle forwhich the X-rays are emitted in the direction of the screen shouldpreferably both be variable as also should the intensity of the X-raysso that the observer is able to choose the combination of parameterswhich gives the optimal sensation with the minimal total output ofradiation.

Advantageously means are provided to give the observer information onthe rate at which X-radiation is being received through the screen. Suchmeans may, for example, be a radiation meter or a badge containing pieceof film which is developed after a known period and compared with astandard.

The controls provided should conveniently enable the observer to adjustthe pulse duration within the range from 0.3 to 3 milliseconds andpreferably less than 1 millisecond. They should also enable therepetition frequency to be adjusted within the range of from to 150pulses per second, e.g. from to 75 pulses per second in order to get thebest results under the referred different conditions. Intensity controlshould, of course, also be provided. Under some conditions the imagemight actually be brighter than given by the unpulsed source since theefliciency of the uoroscopic screen is usually better for intermittentillumination.

FIG. 1 is a schematic diagram of a system which emits constant andpulsatile light. In the drawing:

G.T.-Sylvania type R1131C Glow Tube Pulse- Square pulse input fromTektronix type 161 pulse generator A, B-Manually operated switchesR-Relay operated switch When the cathode (3rd leg) of a Glow Tube isconnected to the plate of the penthode GL6, it generates square lightpulses. When it is connected to the ground it generates a continuouslight, the intensity of which can be controlled by a potentiometer.

With switch A in position 2 and switch B in position 1 GTI emits pulsesof light while GT2 continuous light. On turning B to 2, GTI begins toemit continuous light and GT2 pulsatile light. When A is in position 1the switch R replaces B in the circuit and it automatically switchesfrom position 1 to 2 every 1 or 2 seconds. Under these conditions eachGlow Tube emits alternatively every 1-2 seconds continuous or pulsatilelight.

The mean light enhancement achieved by the use of the present inventionmay be illustrated by employing the Glow Tubes GT, and GT 2 of FIG. 1 toillustrate the response of a uoroscopic screen to continuous and pulsedX-ray beams. However, for actual X-ray tube control, one or both ofthese glow tubes could be replaced by a X-ray tube in the mannerillustrated for the Glow Tubes in FIG. 1 so that the X-ray tube would bepulsed by the variable pulsingcircuit.

Light enhancement is the relationship between the total light energyemitted in pulsed illumination unit time and the total light energyemitted in continuous illumination. This may be charted, as illustratedby FIGS. 2 ad 3, by

making calculations according to the following equation:

enhancement Vp-Pdwhere Vp=the light intensity of one pulse;

Vc=the light intensity of a continuous source;

P=the duration of time between the beginning of a pulse and thebeginning of the pulse which follows it;

Pd=the duration of the pulse The graph of FIG. 2 illustrates the meanlight enhancement calculated at various pulse frequencies using acircuit similar to that of FIG. 1 including a continuous glow tubesource and a pulsed Glow Tube source.

The graph of FIG. 3 illustrates the mean light enhancement calculated atvarious pulse frequencies using a single light source which alternatelyproduced continuous and pulsed illumination in the manner described inconnection with the circuit of FIG. 1. It will be understood that animage intensifier may be used with the apparatus and method of thepresent invention to give additionally improved results.

What is claimed is:

1. A method for minimizing radiation in a fluoroscope while maintainingbrightness perception by achieving brightness enhancement at a frequencyabove the critical fusion frequency range for a fluoroscopic screenwhich includes directing an X-ray beam at a fluoroscopic screen andpulsating said beam at a frequency of 30-150 pulses per second but abovea critical fusion frequency falling within the range of from 20 to 80pulses per second for fluoroscopic screens to form square wave pulseshaving a pulse duration of from 0.3 to 3 milliseconds.

References Cited UNITED STATES PATENTS 2,617,046 11/1952 Douma et al.250-102 2,790,085 4/ 1957 Fransen et al. 250-65 2,905,827 4/1959 Graves250-65 2,937,277 5/ 1960 Euler, Jr. et al. Z50-65 3,277,302 10/ 1966Weighart 250-102 RALPH G. NILSON, Primary Examiner S. C. SHEAR,Assistant Examiner

